Put back default strings on dialogs

But make them work by calling _t in render rather than
getDefaultProps().

Also sort out some 'Warning!' strings

src/components/views/rooms/MessageComposer.js

@@ -127,7 +127,6 @@ export default class MessageComposer extends React.Component {
                     </ul>
                 </div>
             ),
-            button: _t("OK"),
             onFinished: (shouldUpload) => {
                 if(shouldUpload) {
                     // MessageComposer shouldn't have to rely on its parent passing in a callback to upload a file